DE602004001927T2 - Heterostructures of III-nitride semiconductor material for light-emitting devices - Google Patents

Description

BACKGROUND

Field of the invention

The The present invention relates generally to light-emitting Semiconductor devices, and in particular active regions for light emitting III-nitride devices.

description of the related art

light emitting semiconductor devices, such as e.g. Light-emitting Diodes (LEDs) belong to the most powerful Light sources, which are currently available. Material systems, the present in the production LEDs big Brightness, which is about operation enable the visible spectrum of interest are group III-V semiconductors, in particular binary, ternary and quaternaries Alloys of gallium, aluminum, indium and nitrogen, also referred to as III-nitride materials. Typically, III-nitride layers are formed Sapphire, silicon carbide or gallium nitride grown epitaxially.

III-nitride materials become ordinary applied to a light emitting or active region, which several quantum well layers, by barrier layers are separated. The active region is between a p-type region and an n-type region. The p- and n-conducting Lead area the quantum wells in the active area, where the positive and negative charge carriers to Generation of light recombine positive and negative charge carriers (electrons and holes). The brightness of a light-emitting Arrangement is at least partially due to the internal quantum efficiency the arrangement determines which indicates how many light emitting Electron-hole recombination occur in the active region.

each Quantum well layer can be a finite at a given time Number of charge carriers hold. The carrier capacity of a Semiconductor layer hangs depending on how much material is present in the layer; the thicker thus, a quantum well layer is, the more charge carriers can hold this quantum well layer. For III-nitride devices, the quantum well layers are however, typically InGaN, which is compared to other III nitride layers because of the big ones Size of indium atoms and the amount of indium needed to make the quantum well Making light emitting has a poor crystal quality. Another complication is that InGaN is usually at a lower level Temperature is applied as GaN, resulting in reduced crystal quality. Furthermore In-In fluctuation occurs in the InGaN layers, which is the carrier capacity of the light limited to emitting layers. Piezoelectric fields can at reduced oscillator strength for Recombination a reduced overlap of electron and hole wave functions. After all can Defects in the crystalline semiconductor layers of an assembly cause non-radiative recombination of positive and negative charge carriers, what the amount of light that is generated by an arrangement can reduce by the quantum well layers robbed charge carriers become. Thus, the quantum well layers are the crystal quality and the maintain internal quantum efficiency of the light-emitting device, generally thin, and the barrier layers separating the quantum well layers, are generally thick layers which have better crystal quality than the In-containing ones Have quantum well layers.

Y T Rebane et al, Light emitting diode with charge asymmetric resonance Tunneling, Physics Status Solidi (a), Vol. 180, pp. 121-126 (2000), describes light-emitting diodes, wherein an electron emitter via a Barrier is connected to the active area. The electrons tunnel through the barrier into the active quantum well, where they are generating recombine light with holes.

T C Wen et al, InGaN / GaN tunnel injection blue light-emitting diodes, IEEE transactions on Electron Devices, Vol. 49, No. 6, pp. 1093-1095 (2002) Light emitting diodes, wherein an electron emitter via a Barrier is connected to the active quantum well, wherein the active Quantum well has a multi-quantum well (MQW) structure, which from several narrow quantum wells which are separated by tunneling barriers.

P Battacharya et al, Tunneling laser Lasers: a new that of lasers with reduced hot carrier effects, IEEE Journal of Quantum Electronic, Vol. 32, pages 1620-1629 (1996), describes laser diodes, wherein an electron emitter via a Barrier is connected to the active quantum well, wherein the active Quantum well a multi-quantum well (MQW) structure which consists of a plurality of narrow quantum wells, which by Durchrunungsbarrieren are separated.

SUMMARY

According to embodiments of the present invention, heterostructure designs by which the number of carriers present in the quantum well layers of the active region can be increased for light emitting III-Nit rid arrangements, such as light-emitting diodes disclosed. In a first embodiment, a reservoir layer is provided together with a barrier layer and quantum well layer in the active region of a light-emitting device. In some embodiments, the reservoir layer is thicker than the barrier layer and quantum well layer, has a larger indium composition than the barrier layer and a lower indium composition than the quantum well layer. In some embodiments, the reservoir layer is disposed adjacent the quantum well layer and is graded into "funnel" carriers into the quantum well layer.

In a second embodiment For example, the active region of a light-emitting device is a superlattice from alternating quantum well layers and barrier layers. In some embodiments the barrier layers are so thin that charge carrier tunnel through quantum well layers through a barrier layer can.

DESCRIPTION OF THE DRAWING

embodiments The invention are illustrated in the drawings and will be described in more detail below. Show it:

1 A light-emitting device according to a first embodiment of the present invention;

2 An example of the conduction band edge energy of the layers of 1 arrangement shown;

3 An example of the conduction band edge energy of the layers of 1 arrangement shown taking into account polarization fields;

4 A light-emitting device according to a second embodiment of the present invention;

5 to 8th Examples of the conduction band edge energy of the layers of 4 arrangement shown;

9 A light-emitting device according to a third embodiment of the present invention;

10A to 10C Examples of the conduction band edge energy of a thin quantum well layer, a thick quantum well layer and a superlattice as the active region;

11 An energy band diagram considering polarization fields for the in 9 superlattices shown;

12 - An exploded view of a light-emitting device, including a housing.

DETAILED DESCRIPTION

In accordance with embodiments of the present invention, heterostructure designs that can increase the number of carriers present in the quantum well layers of the active region are disclosed for III-nitride light emitting devices, such as light emitting diodes. Ill-nitride semiconductor layers as used herein, compounds represented refer to, by the general formula Al _x Ga _y In _1-xy N (0 ≤ x ≤ 1, 0 ≤ y ≤ 1, 0 ≤ x + y ≤ 1) which may further contain Group III elements such as boron and thallium and in which part of the nitrogen may be replaced by phosphorus, arsenic, antimony or bismuth.

1 shows a transverse crack of a light-emitting device according to a first embodiment of the present invention. At the in 1 arrangement shown are one or more n-type layers 11 on a substrate 10 applied epitaxially. substratum 10 For example, it may be represented by a sapphire, SiC, GaN, or other suitable substrate. N-conducting layers 11 For example, they may include a buffer layer, a contact layer, an undoped crystal layer, and n-type layers of different composition and doping concentration. Then, on the n-type layers 11 an active area 18 educated. After that, be on the active area 18 one or more p-type layers 15 intended. P-type layers 15 For example, they may comprise a confinement layer, a contact layer, and other p-type layers of different composition and doping concentration. Subsequently, one or more p-metal layers 16 which form the electrode or the contact to p-type layers 15 form on the p-type layers 15 applied. A portion of the active region and the p-type layers is removed to one of the n-type layers 11 expose. On the exposed part of the n-type layers 11 becomes one or more n-metal layers 17 applied to make contact with the n-type layers of the device.

The active area 18 includes quantum well layers 14 , Barrier layers 13 as well as reservoir layers 12 , 2 FIG. 12 shows an example of a conduction band edge energy diagram for some of the layers of FIG 1 illustrated arrangement. As in 2 shown have reservoir layers 12 a larger band gap than quantum wells 14 and a smaller band gap than barrier layers 13 and the device layers 11 . 15 outside the active area. The quantum well layers 14 and the reservoir layers 12 can be made from InGaN and the barrier layers 13 Al _x Ga _y In _1-xy N (0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ x + y ≦ 1). As the indium content in an InGaN layer decreases, the bandgap increases; thus, the reservoir layers 12 less indium than the quantum well layers 14 and more indium than the barrier layers 13 contain. The quantum well layers 14 For example, an indium composition can range from 0% (for example, if the barrier layers are AlGaN) to about 30%, and usually provide an indium composition in a range of between about 4% and about 25%. Barrier layers can have an indium composition in a range between 0% and 15% and typically have an indium composition in a range between 0% and 5%. Reservoir layers may consist of GaN, InGaN or AlInGaN. In one embodiment, the reservoir layers are InGaN having an indium composition in a range between 0% and about 25%.

barrier layers 13 are thin enough to allow charge carriers through barrier layers 13 from the reservoir layers 12 to quantum well layers 14 can tunnel. The barrier stories 13 may have a thickness between about 5 angstroms and about 200 angstroms (10 angstroms = 1 nanometer, nm) and often have a thickness of 20 angstroms or less; the quantum well layers 14 may have a thickness between about 5 angstroms and about 100 angstroms and often have a thickness of about 25 angstroms; and the reservoir layers may have a thickness between about 5 angstroms and about 500 angstroms and often have a thickness greater than about 25 angstroms. The reservoir layers 12 and the barrier stories 13 may or may not be doped with a p- or n-type dopant. In one embodiment, the reservoir layers are doped with an n- or p-type dopant at a doping concentration between about 5 × 10 ¹⁶ cm and about 5 × 10 ¹⁹ cm ^-3 . In one embodiment, the reservoir layers are 12 with the dopant of the same conductivity type as the other layers in the active region 18 doped.

The reservoir layers 12 see a store of charge carriers in the immediate vicinity of the quantum wells 14 in front, which by barrier layer 13 easily into the quantum wells 14 can tunnel. The reservoir layers 12 lead the quantum well layers 14 Charge carrier, so that as soon as a charge carrier leaves a quantum well layer, for example by radiative recombination, immediately from the reservoir layer 12 another charge carrier is supplied to replace it. Because the reservoir layers 12 less indium than the quantum well layers 14 have, have the reservoir layers 12 a better crystal quality, reducing the reservoir layers 12 Hold charge carriers with a long cooldown. The composition and thickness of quantum well layers 14 , Barrier layers 13 and reservoir layers 12 can be chosen so that τ RV >> τ DO >> τ W (1) where τ _{RV is} the residence time of a charge carrier in the reservoir layer 12 , τ _DO the time required for a carrier to move from reservoir layer 12 to quantum well layer 14 to tunnel and τ _W the residence time of a charge carrier in quantum well layer 14 before the charge carriers recombine to produce light. Since the residence time in reservoir 12 much longer than the residence time in quantum well layer 14 is, finds in reservoir 12 a constant supply of charge carriers takes place to fill gaps in quantum well 14 fill.

The probability of barrier layers 13 To tunnel through can be maximized by the composition and thickness of reservoirs 12 and quantum wells 14 is chosen so that the energy levels of the states, which charge for loading of carriers in reservoirs 12 and quantum wells 14 exist, are almost equal. When the energy levels of the states are nearly equal, there is a high probability that a charge carrier will be between a reservoir 12 and a quantum well 14 tunnels. The tunneling probability can also be maximized by exploiting the polarization fields inherent in III-nitride materials. 3 FIG. 12 shows an example of a conduction band edge energy diagram for some of the layers of FIG 1 arrangement, taking into account present in III-nitride layers polarization fields. The polarization field distorts the energy band diagram so that the conduction band in reservoir layer 12 at the corner 30 less than in the remainder of the reservoir layer 12 is. Likewise, the conduction band is in quantum well layer 14 at the corner 32 lower than in the remainder of quantum well layer 14 , Because carriers have a tendency to corner themselves 30 because the conduction band is lower than in the rest of the reservoir 12 is, the charge carriers to the quantum well 14 "pushed".

Although 1 a reservoir 12 on both sides of each quantum well layer 14 shows, different configurations of reservoirs in the active area can be used. First, two quantum well layers or more between Re be arranged servoirs. For example, the active region may be provided such that one reservoir over the n-type layers, a first barrier layer over the reservoir, a first quantum well over the first barrier, a second barrier over the first quantum well, a second quantum well over the second barrier , a third barrier is placed over the second quantum well, and then a second reservoir is placed over the third barrier. Second, reservoirs can only be used at the center of the active area. The outermost layers of the active region may be barrier layers or quantum wells and need not, as in the 1 illustrated arrangement, be represented by reservoir layers. Third, different reservoir layers may have different compositions or thicknesses. The reservoir layers need not have identical composition and thickness as in the 1 have shown arrangement. Fourth, the placement of reservoir layers in the active area need not be symmetric. For example, an array may have a reservoir adjacent to the n-type layers on one side of the active area and then a barrier layer or quantum well adjacent to the p-type layers on the other side of the active area.

4 shows a light-emitting device according to a second embodiment of the present invention. At the in 4 The arrangement shown are the quantum well layers 14 from the reservoir layers 12 not separated by barrier layers. barrier layer 13 can, as in 4 shown, quantum well layers 14 separate from each other, or the quantum well layers 14 can be separated by another reservoir layer.

5 to 8th show examples of conduction band edge energy diagrams for some of the layers of FIG 4 illustrated arrangement. As in the 5 to 8th can represent, reservoir layers 12 have a graded composition. The term "graded," as used herein in describing the composition or doping concentration in a layer in an array, is to be interpreted as referring to a structure in which a change in composition and / or doping concentration is otherwise is achieved as in a single step change in composition and / or doping concentration In an example, as in 7 As shown, the graded reservoir is represented by a stack of layers, each of the layers having a different composition and / or doping concentration than any adjacent layer. Should the layers have a resolvable thickness, the graded region is known as a "step-graded" or "index-graded" region. In the boundary region where the thickness of individual layers approaches zero, the graded reservoir region is known as a continuously graded region. The layers forming the graded reservoir region may be provided to have different profiles in composition and / or doping concentration vs. Thickness including, but not limited to, linear ascents, parabolic ascents, and power-law ascents. Similarly, graded reservoir regions are not limited to a single gradation profile, but may include parts having different gradation profiles and one or more parts having regions having a substantially constant composition and / or doping concentration.

As in the 5 and 6 can represent reservoir layers 12 be provided with a linear, monotonic gradation in the composition. At the in 5 The arrangement shown is the composition of indium in reservoir 12 of the percentage of indium in n-type layers 11 , typically zero, to an indium composition which is less than the indium composition in the quantum well layers 14 is, graded. The indium composition then takes in quantum well layer 14 to. As in the out 6 shown arrangement, the indium composition in reservoir layer 12 of the indium composition in the n-type layers 11 up to the indium composition in quantum well 14 without an abrupt step change in the indium composition between the graded reservoir region 12 and the quantum well layer 14 be graded continuously. Likewise shows 6 in that the gradation in the reservoir adjoins the n-type layers 11 not with the gradation of the reservoir area adjacent to the p-type layers 15 must be identical or symmetrical to this.

At the in 7 illustrated arrangement is reservoir 12 represented by a series of layers, each having a different indium composition. In reservoir 12 adjoining the p-type layers 11 the composition of indium is increased upon application of each of the layers. The increase or decrease in indium composition need not be the same for each layer in the stepwise graded reservoir. At the in 8th arrangement shown are reservoirs 12 provided with non-linear, monotonic gradation in the composition.

The in the 5 - 8th The grading schemes of the reservoir layer shown and described above are of a low indium composition in the n-type and p-type Layers adjacent to the active region are graded to a high indium composition in the region of the reservoir adjacent to the quantum well layers in the active region. The gradation is selected to provide a "funnel" which directs charge carriers into the quantum well layers, Moreover, the presence of graded reservoir layers immediately adjacent to the active region increases the amount of material provided to hold charge carriers, thereby increasing the charge Number of the Quantumopfschichten available charge carriers is increased.

9 shows a light-emitting device according to a third embodiment of the present invention. At the in 9 The arrangement shown form the quantum well layers 14 and the barrier stories 13 in the active area 18 a supergrid. The quantum well layers and the barrier layers may have the same indium composition as those described above with reference to 1 have described arrangement. In a superlattice, the quantum well layers may have a thickness between about 5 angstroms and about 50 angstroms and often have a thickness between about 20 angstroms and about 40 angstroms. The barrier layers may have a thickness between about 5 angstroms and about 110 angstroms and often have a thickness less than 25 angstroms. The barrier layers are thin enough to allow charge carriers through barrier layers 13 in the active area 18 between quantum well layers 14 can tunnel. At a certain barrier thickness, the tunneling rate depends on the width of the quantum well. As the thickness of the quantum well decreases, the tunneling rate increases as the difference in energy level between the quantum well and the barrier decreases. In some embodiments, the thickness of each quantum well in the active region may be appropriately selected at the desired whitening rate in the vicinity of that quantum well. This in 9 shown superlattice as an active area can with the in the 1 to 8th shown reservoir layers are used. For example, the in 9 illustrated arrangement comprise reservoir layers on one or both sides of the superlattice and / or one or more reservoir layers within the superlattice.

Are the barriers thin enough to allow charge carriers, as in 9 shown to be able to tunnel between quantum well layers, the quantum well layers are "coupled", which means that the active region 18 how a single quantum well layer behaves, but not like several discrete, thin quantum well layers. This coupling can offer several advantages. First, since carriers tunnel between quantum well layers, the carriers can be found in each of the quantum wells, with the carriers exposed to more active material. This is an improvement over a conventional, active region with thick barrier layers. As a result, there are more charge carriers for recombination in the quantum well layers of the active region, which can increase the light output of the device.

Second, in a superlattice, the active region may be a lower constraint penalty than in a conventional, active region with thick barrier layers. The limitation disadvantage is in the 10A - 10C which line band edge energy diagrams for a thin quantum well ( 10A ), a thick quantum well ( 10B ) and a superlattice as the active area ( 10C ) demonstrate. line 121 represents an energy level which is a charge carrier in the 10A must occupy the apparent quantum well. Because the quantum well of 10A is thin and the carrier is confined to a small space is energy level 121 high. The opposite is the energy level 122 in the wide quantum well of 10B much lower than energy level 121 in 10A because the charge carrier in the wide quantum well of 10B is less limited. 10C shows an energy level 123 which must occupy a charge carrier in a superlattice as the active region. One can think of the superlattice as acting like a broad quantum well with several "pillars" in it, where the charge carriers are "located" in one of the pots and are therefore better distributed compared to a thin quantum well. The presence of the barrier layer "pillars" increases the energy of a pot with the same width of the active region.The energy level determines the color of the emission from the array.As the composition of indium in the quantum well layers also affects the emission color, a reduction of the energy level As described above, the crystal quality of the layer typically suffers as the proportion of indium in a layer increases, and poor crystal quality can cause reduced efficiency. which provide a superlattice as active regions, be more efficient while emitting light of the same color as uncoupled quantum well devices with a higher indium composition.

Third, a superlattice as an active region can reduce the effects of polarization fields. 11 shows an energy band diagram for a superlattice as the active area. The in a Übergit Thin barrier layers used as the active region can reduce the effects of polarization fields by allowing for spatially indirect recombination, which can increase the amount of light generated by an active region. Because of the polarization field which "skews" the bottom of each quantum well, carriers tend to be in the areas 110 to collect each quantum well layer. Since the barrier layers are thin in a superlattice as the active region, charge carriers can, as shown by arrow 114 shown to be able to recombine via barrier layers.

12 is an exploded view of a packaged, light-emitting device. A heat dissipation slug 100 gets into a cast lead frame 106 used. The cast support 106 For example, a filled plastic material is molded around a metal frame that provides a current path. "Slug" 100 can be an optional reflector shell 102 exhibit. The LED chip 104 , which may be represented by one of the arrangements described above, is provided by a thermally conductive mounting base 103 directly or indirectly to "slug" 100 appropriate. It can also be an optical lens 108 be arranged.

On Reason of the present, detailed description may be of Expert modifications are made without going by the Invention as in the claims defined, depart. The scope of the invention is therefore not on the illustrated and described, specific embodiments to restrict.

Claims (17)

Light emitting device comprising: a substrate ( 10 ), an area ( 11 ) of the first conductivity type which is above the substrate ( 10 ), an active area ( 18 ) overlying the region of the first conductivity type, the active region comprising: a quantum well layer ( 14 ), which contains Al _x Ga _y In _1-xy N (0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ x + y ≦ 1) and has a first band gap, a barrier layer ( 13 ) and a reservoir layer ( 12 ), which contains Al _x Ga _y In _1-xy N (0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ x + y ≦ 1), has a second band gap and the quantum well layer ( 14 ) Supplies charge carriers and a region of the second conductivity type ( 15 ) disposed over the active region, wherein the first band gap is less than the second band gap, characterized in that a composition of at least a portion of the reservoir layer (10) 12 ) of a first indium composition in a first part of the reservoir layer adjoining one of the regions of the first conductivity type ( 11 ) and second conductivity type ( 15 ) to a second composition in a second part of the reservoir layer adjoining the quantum well layer ( 14 ) or the barrier layer ( 13 ) and the indium concentration in the first indium composition is lower than that in the second indium composition. A light emitting device according to claim 1, wherein: the barrier layer ( 13 ) has a third bandgap and the third bandgap is greater than the second bandgap. A light emitting device according to claim 1, wherein: the quantum well layer ( 14 ) has a first indium composition, the reservoir layer ( 12 ) has a second indium composition and the reservoir layer ( 12 ) less indium than the quantum well layer ( 14 ) contains. A light-emitting device according to claim 3, wherein: the barrier layer ( 13 ) has a third indium composition and the reservoir layer ( 12 ) more indium than the barrier layer ( 13 ) contains. A light-emitting device according to claim 1, wherein the barrier layer ( 13 ) between the quantum well layer ( 14 ) and the reservoir layer ( 12 ) is arranged. A light emitting device according to claim 1, wherein: the quantum well layer ( 14 ) has a first thickness, the reservoir layer ( 12 ) has a second thickness and the second thickness is greater than the first thickness. A light-emitting device according to claim 6, wherein: the barrier layer ( 13 ) has a third thickness and the first thickness is greater than the third thickness. A light emitting device according to claim 1, wherein: the barrier layer ( 13 ) has a thickness between 0.5 nm and 5.0 nm, the quantum well layer ( 14 ) has a thickness between 0.5 nm and 10.0 nm and the reservoir layer ( 12 ) has a thickness between 0.5 nm and 25.0 nm. The light emitting device of claim 1, wherein: the reservoir layer ( 12 ) provides a plurality of sublayers, each sublayer having an indium composition, a sublayer having the least indium composition adjacent to the first conductivity type region or the second conductivity type region, and a sublayer having the highest indium composition adjacent to the quantum well layer or the barrier layer is. A light-emitting device according to claim 1, wherein the gradation is monotone. A light-emitting device according to claim 1, wherein the composition in a profile consisting of the group of linear, graduated and parabolic profiles is graded. A light-emitting device according to claim 1, wherein the reservoir layer has a first part with a constant composition and a second part having a graded composition. The light emitting device of claim 1, further comprising: a first contact ( 17 ) which is connected to the region of the first conductivity type ( 11 ), a second contact ( 16 ) which is connected to the region of the second conductivity type ( 15 ), a mounting base, which is connected to the first and the second contact, and a lens ( 10 ), which is disposed above the substrate. A light-emitting device according to claim 1, wherein the active region ( 18 ) a superlattice of alternating quantum well layers ( 14 ) and barrier layers ( 13 ), wherein the barrier layers have a thickness such that charge carriers can tunnel through the barrier layers between quantum well layers. A light-emitting device according to claim 14, wherein at least one barrier layer has a thickness between 0.5 nm and 11.0 nm and at least one quantum well layer has a thickness between 2.0 nm and 4.0 nm. A light-emitting device according to claim 14, wherein: at least a barrier layer has a thickness between 0.5 nm and 2.5 nm and at least one quantum well layer has a thickness between 2.0 nm and 2.5 nm. A light-emitting device according to claim 1, wherein this further comprises: a first contact, which with the region of the first conductivity type connected is, a second contact, which with the area of the second conductivity type connected is, a mounting base, which with the first and second contact is connected, as well a lens which over the Substrate is arranged.